An image forming apparatus of an ink-jet type (hereinafter, it may be called an ink-jet image forming apparatus), for example, an ink-jet printer, has such a simple constitution that allows the apparatus to be miniaturized, while keeping manufacturing cost of the apparatus low. Moreover, the ink-jet image forming apparatus forms an image by jetting liquid ink droplets from a nozzle. Here, the ink droplets are directly absorbed and adhered onto a recording medium, such as paper, a film, and cloth, thus requiring no further processing. Therefore, a color image can be easily formed by using a color ink.
However, it is important for the ink used in the image formation by the ink-jet image forming apparatus that the ink can penetrate into the recording medium and is able to dry on the recording medium. As the ink used in the image formation by the ink-jet image forming apparatus, pigment-based ink and dye-based ink are mainly used. It is well known that the pigment-based ink is inferior to the dye-based ink, in terms of drying and penetrating properties, but the pigment-based ink is better than the dye-based ink in terms of the coloring and color thickness.
For example, recording paper, which is especially for use of the ink-jet printer (recording paper for the ink-jet printer), may be used in order to form an image by the ink-jet image formation apparatus. This can form an image free from color-mixing and bleeding even if printing is carried out with a pigment-based black ink, which is inferior in terms of the penetrating and the drying properties. This is because the recording paper for the ink-jet printer has been subjected to treatments to improve its absorbing and fixing properties with respect to the ink adhered on the recording paper. But, this leads to a higher recording medium cost. If common photocopying paper is used as the recording medium in order to reduce the cost of the recording medium, use of an ink having inferior drying and penetrating properties results in such printing that ink droplets adjacently adhered on the recording medium are merged together, thereby causing the mixing of colors and bleeding. Especially in a region where a color dot area and a block dot area border on each other, black lines and characters becomes indistinct due to the mixing of colors and bleeding.
To solve the forgoing problems, Japanese Un-Examined Patent Application, Tokukaihei, No. 7-149036 (published on Jun. 13, 1995) and Japanese Un-Examined Patent Application, Tokukaihei, No. 8-197831 (published on Aug. 6, 1996) disclose prior arts in which results of printing of the pigment-based black ink having the drying and penetrating properties of, which has the inferior drying and penetrating properties, are improved (a) by printing the black ink so as to overlap on the color ink, which has a superior penetrating property, and (b) by lowering a density of black dots in an image region, so as to make a gap between the black dots so as to print a dot of a color ink in the gap. Those prevent the mixing of colors, the bleeding, improper printing due to rubbing on the recording medium before the ink is dried, and staining between the black ink and the color ink.
However, in case the black ink and the color ink are overprinted by using an ink head section that has a conventional ink head arrangement (an arrangement having a black ink head in ends of an order, for example, an arrangement shown in FIG. 11, where a black ink head (K), a cyan ink head (C), a magenta ink head (M) and a yellow ink head (Y) are disposed in this order, or an arrangement shown in FIG. 12, in which a cyan ink head (C), a magenta ink head (M), a yellow ink head (Y), and a black ink head (K) are placed in this order), the black ink and the color inks are printed in different orders, depending on in which way the ink head section scans (that is, depending on the scanning direction of the ink head section). For this reason, an image formed in this manner has “blackness” (coloring) of black color that looks different depending in which way the ink head section scans. In a region where inks are printed in one way of the scanning direction, the black ink is printed on the color ink, while the color ink is printed on the black ink in a region printed in the other way of the scanning direction. Therefore, if printing is carried out by the ink head section, which has the conventional ink head arrangement, scanning in the two ways of the scanning direction, the blackness in a printed black image is different between the respective ways of the scanning direction of the ink head section. The difference of the blackness causes a problem of deterioration of image quality.
On the other hand, it seems that the foregoing problem is not caused when the density of the black dots in the black image area is lowered so as to make a gap between the black dots of the black ink, so that the color dot of the color ink can be printed in the gap, that is, the printing is carried out in such a manner that the black ink and the color ink are printed adjacent to each other, even if the ink head section having the conventional ink head arrangement is used.
However, in reality, both the black ink and the color ink penetrate and spread into the recording medium. Thus, different blackness of the black image is caused depending on whether the black ink is printed before the color ink or after the color ink, just like the case where the black ink and the color ink are overprinted. Therefore, the quality of the image is deteriorated when the printing is carried out by the ink head section, which is in the conventional arrangement and scans in two ways, even if the density of the black dot is reduced so as to make the gap between the black dots of the black ink so that the color dot of the color ink can be printed in the gap.
Therefore, it is necessary to fix the order of the black ink and the color inks to be printed in each scan of a carriage, in order to have no difference in the blackness of the black printing. But, with the ink head section having the conventional ink head arrangement, the printing must be carried out only in one way of the scanning direction in order to fix the order of the ink. For this reason, the printing cannot be performed in the two ways in the scanning direction, and the printing time cannot be shortened.
Moreover, Japanese Un-Examined Patent Application, Tokukaihei, No. 7-195715 (published on Aug. 1, 1995) and Japanese Un-Examined Patent Application, Tokukaihei No. 11-216882 (published on Aug. 10, 1999) disclose how to shorten the printing time while maintaining the image quality. According to the two applications, a specific ink head arrangement (for example, an arrangement shown in FIG. 17) can fix the order of the inks to print, even if the printing is carried out in the two ways in the scanning direction in an ink-jet image forming apparatus, thereby making it possible to form an image with no difference in the blackness between each scan of the carriage.
However, if the prior arts recited in the two applications are combined together, the ink heads increases in number from 4 to 6 or 7 (this means the number of ink heads is almost doubled), thereby resulting a higher cost. Thus, it is hard to apply those prior arts in an ink-jet image forming apparatus having a low price.